Electrostatic precipitation is fundamentally different from most other methods of particulate collection in that the forces used to separate the particles from the mainstream are electrical, as opposed to mechanical. These forces act only on the particles themselves and not the entire gas stream resulting in less resistance to mainstream flow.
Electrostatic precipitation consists of three fundamental steps in removing particulate from a gaseous medium. First, the particles are charged to the same sign. Then they are gathered at collection surfaces by means of the forces induced by the electric charge, specifically the Coulomb, image and space charge forces. Finally, the collected mass is removed from the system either by mechanical or thermal means.
The charging is accomplished by applying a high electric potential between two electrodes; one with an extremely small radius of curvature such as a sharp point called the emitting electrode and another with a much larger radius called the attractor electrode. The high intensity electric field thereby created adjacent to the sharp point causes ionization due to collision between fast moving electrons with the gas molecules. Thus, free electrons in the intense electric field act as ionization agents to electrically charge the gas molecules to produce an active or ionization region, known as corona glow. It serves as a copious electron source. If the fine electrode is at negative polarity, the ions become negatively charged. These negative ions propagate out towards the attractor electrode filling the region outside the corona, known as the passive zone, creating a "space charge" with about 10.sup.8 ions per cubic centimeter at ordinary pressures. As the gas with suspended particles is passed through the passive region of the space-charge field, the particles are subject to an intense barrage of negative ions becoming rapidly negatively charged. Thus the suspended particles acquire the same polarity as the gaseous ions.
It is posssible to have field emission without a corona and momentum exchange may also occur between the free electrons and the gaseous molecules. The charged molecules are, in turn, accelerated away from the emitting electrode causing a noticeable flow of the gas called "electric wind." In U.S. Pat. No. 3,767,258 to Smith, et al., the phenomenon of electric wind is employed to charge the particulate matter in an exhaust stream, however, the surfaces surrounding Smith, et al.'s charging needles are nonconductive thereby preventing the establishment of a corona. Electric wind systems of electrostatic precipitation are generally not accepted as efficient as corona based systems.
In a corona system, as a solid particle enters the space-charge region, it immediately acquires a charge as the gas ions travel along the lines of force in the electric field and converge with the particle. The charging process continues until a sufficient repelling field created by the accumulation of charge has been built up to prevent the ions from reaching the particle. This phenomenon is called saturation.
Particle charging may be carried out by either positive or negative corona discharge. Positive corona discharge is procured by applying a positive voltage to the emitting electrode whereas a negative corona is obtained with a negative emitting electrode. Negative corona is predominantly used in electrostatic precipitation as it is characteristically more stable than a positive corona (e.g. see U.S. Pat. No. 1,931,436 to Deutsch). It is suitable for a gaseous medium which is electronegative. Oxygen, water vapor, carbon dioxide, etc. prevailing in the exhaust of a diesel engine are electronegative gases.
The physical difference between positive and negative corona is that in negative corona, positive ions are attracted to the corona needle and negative ions and electrons are drawn to the attractor electrode. In positive corona, negative ions and electrons are attracted to the corona needle while positive ions propagate to the attractor. The initial source of electrons in positive corona is the electron emission from gas molecules by ultraviolet light quanta while the corona electrode itself is the chief source of electrons in negative corona discharge.
In U.S. Pat. No. 4,435,190 to Taillet, et al., a negative corona system is employed to charge a flow of moist air which is accelerated to supersonic speed resulting in the formation of microparticles of ice. Ions trapped by these microparticles are freed by sublimation to create a flow of space charge which is impelled into the flow containing particulate matter. The particulate matter acquires the desired charged state as it passes through the space charge field. This scheme of using microparticles of ice as ion carriers is to alleviate the reduced efficiency of corona discharge in high temperature applications. (eg. Hot dusty gases at 900.degree. C.).
A major problem incident to many electrostatic systems is the contamination of the charging system by the particulate matter in the exhaust stream (results in arc-overs). Taillet, et al. avoids this problem by using an intermediate gas containing microparticles of ice to carry the charge to the exhaust stream. Smith, et al. employs an outwardly directed auxiliary gas flow surrounding his charging needles. The Taillet, et al. system, though complex, is not as efficient as a particle charging system which employs corona discharge. The Smith system does not employ a corona and moreover requires a substantial gas flow to keep the particulate matter away from the charging needles and to alleviate the vacuum which is created by his shielded charging system--which is oriented with its open face directed in the downstream direction.
Others have attempted to prevent contamination through the use of various types of circulating flows of air on other gases about certain surfaces exposed to the particulate matter, e,g. see U.S. Pat. Nos. 3,238,702 to DeSeversky; 1,994,259 to Thorne; and 4,406,119 to Kamiya et al. However, no known contamination prevention system has been devised which enables long term operation of an electrostatic particulate removal system for diesel engines.